This invention relates to a bioactive cement for bonding or fixing an implant material used in orthopedic and dental fields.
In the orthopedic field, a bone defect due to fracture, bone tumor, or any other disease is dealt with. In some cases, a part of a bone is resected in a surgical operation. In the dental field, a defect of a jawbone may result from extraction of a tooth, a Riggs' disease, and so on. In order to repair such bone defect and to reconstruct the part which has been resected, use is made of an implant material comprising a substance selected from metal, ceramics, and crystallized glass.
It is desired that such implant material is quickly and adaptively embedded and fixed in a repair part to be repaired. For this purpose, the implant material must be ground or worked into a shape adapted to the repair part. However, it is extremely difficult to perform such grinding or working with a high precision.
In view of the above, a biocement is generally used together with the implant material in order to bond and fix the implant material to a living bone. For example, in the orthopedic field, a polymethylmethacrylate (PMMA) cement has been widely used. In the dental field, use has been made of a zinc phosphate cement or a carboxylate cement.
The above-mentioned biocements of various types can make a strong bond with the implant material. However, those biocements may possibly be loosened from the living bone and frequently induce an inflammatory reaction in a surrounding tissue.
Under the circumstances, various proposals have been made of the improvement of the biocement which contains a bioactive substance as a filler so as to provide a chemical bond with the living bone. For example, Japanese Patent Publication No. 42384/1979 discloses a biocement comprising a combination of polymethylmethacrylate (PMAA) and K.sub.2 O--Na.sub.2 O--CaO--MgO--SiO.sub.2 --P.sub.2 O.sub.5 crystallized glass powder.
Japanese Patent Prepublication No. 503148/1987 discloses another biocement comprising a combination of 2,2-bis[4-(3-methacryloxy-2-hydroxy propoxy)phenyl]propane (hereinafter referred to as Bis-GMA) base monomer and apatite powder with bioglass powder added as an optional component.
However, the conventional biocements described above are not yet satisfactory in bonding condition with the living bone, bonding strength, mechanical strength, and chemical stability of a hardened cement body itself which is obtained after completion of a hardening process.
For example, the biocement disclosed in Japanese Patent Publication No. 42384/1979 is disadvantageous in that the bonding strength with the living bone is insufficient. As a result of thorough investigation, the present inventors have found out that, once this cement is hardened, body fluid such as cerebrospinal fluid, lymph, and saliva is not allowed to filtrate into the interior of the hardened cement body and therefore the crystallized glass powder can not exhibit bioactivity. It has also been found out that this is because the biocement uses the monomer (methylmethacrylate, hereinafter abbreviated to MMA) which is substantially non-hydrophilic.
In addition, the hardened cement body itself has a reduced mechanical strength because of two-dimensional polymerization of the above-mentioned MMA.
Furthermore, the biocement disclosed in Japanese Patent Publication No. 42384/1979 comprises, as a filler, the crystallized glass powder containing alkali components such as Na.sub.2 O and K.sub.2 O. Accordingly, chemical durability is not excellent and bioactivity is insufficient.
On the other hand, the biocement disclosed in Japanese Patent Prepublication No. 503148/1987 employs, as a hardening agent, Bis-GMA which is highly hydrophilic. In accordance with our findings described above, it is supposed that the body fluid can easily filtrate in the interior of the hardened cement body and therefore the biocement can form a chemical bond with the living bone. However, the present inventors has practically confirmed that a sufficient bonding strength can not be achieved with this biocement. This is because the biocement uses a filler comprising apatite which has low bioactivity. In case when the apatite powder is exclusively used as the filler, the bonding rate is rather slow and the bonding strength is weak. When the bioglass powder is additionally used, the bonding rate is increased. However, a silica gel layer, which is thick and fragile, is formed on the surface of the bioglass powder because Na.sub.2 O contained in the glass is easily precipitated in the form of Na.sup.+ ion. As a result, a strong bond with the living bone is difficult to obtain. In addition, the precipitated Na.sup.+ ion increases the pH value of the body fluid. Accordingly, the surrounding tissue may possibly be adversely affected. Furthermore, the bioglass has a low chemical durability. If this biocement is embedded in contact with the body fluid for a long time, the glass powder is broken. This results in deterioration of the mechanical strength of the hardened cement body.